In a conventional fuel cell system (see Japanese Patent Application Laid-Open No. H09-259909, for example), air, fuel gas, and coolant are each fed to the system's fuel cell stacks through a single feeding pipe and are also merged into a single exhaust pipe upon being expelled from the system. In this case, the volume of coolant is controlled in accordance with a flow volume parameter provided to the single pipe on each of the feed and exhaust sides.
Generally, these types of conventional fuel cell systems focus primarily on the heat generation of the system as a whole and do not take into consideration the efficiency of individual cell sets within the cell system. Individual cell sets within a fuel cell system however may deteriorate at different rates over time. As a result, certain cell sets may become less efficient and thus generate more heat than others during operation of the cell system. When this occurs, the heating value of the less efficient cell sets increases such that it becomes more difficult to maintain or control the temperature of the fuel cell stacks within a specified or even acceptable tolerance range.